Method and apparatus for distillation

ABSTRACT

A composite membrane structure is used in a distillation apparatus and method. The composite membrane separates a distilland and a distillate, such as salt water and fresh water. The composite membrane comprises a thin lyophobic microporous layer or membrane and a thin lyophilic layer or membrane. Evaporation and condensation takes place within the micropores of the lyophobic membrane. The lyophilic layer prevents intrusion of distilland into the pores of the lyophobic layer.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for distillationand in particular, to one utilizing a composite membrane through whichevaporation and condensation takes place.

Conventional distillation requires heating the distilland to the boilingpoint of one or more of the constituents of the distilland and thenrecovering and condensing the resulting vapor to a liquid, thedistillate.

Distillation has also been accomplished through the use of a thin,porous, lyophobic membrane separating the distilland, typically anaqueous solution such as salt water, from the distillate, such as purewater.

The terms lyophobic and lyophilic are employed here to describe thewettability of a liquid on a solid surface. Lyophobic describes anon-wetting behavior of a liquid such that the contact angle between aliquid and a solid is greater than 90° so that the liquid tends to ballup and run off the surface easily. Lyophilic describes a wettingbehavior of a liquid such that the contact angle between a liquid and asolid is less than 90° and the liquid spreads over the solid easily.

The contact angle or wettability depends on the relative attractionbetween the solid and liquid and between the molecules of the liquidthemselves. If the attraction of the liquid for the solid is more thanhalf that for itself, the contact angle is less than 90° and wetting issaid to occur and the liquid is said to be lyophilic with respect to thesolid. A liquid is lyophobic with respect to a solid when the attractionof the liquid for the solid is less than half that for itself therebycreating a contact angle greater than 90°.

When the liquid is water, the terms hydrophobic and hydrophilic aregenerally substituted for the more inclusive designations.

Due to the properties of the non-wetting lyophobic material at lowpressures, liquid is prevented from entering within the pores of thelyophobic membrane. If the distilland has a higher equilibrium vaporpressure than the distillate, evaporation takes place at the hotterdistilland side of the lyophobic pores and condensation occurs at thecooler, distillate liquid interface of the lyophobic membrane pores.Thus, in the case of the aqueous solution, in an effort to establishvapor pressure equilibrium, a net flux of water vapor thus passesthrough the pores of the hydrophobic membrane. One example of suchthermal membrane distillation is described in U.S. Pat. No. 3,340,186.

This type of distillation process should be contrasted with reverseosmosis processes which also use porous membranes. Reverse osmosis doesnot involve an evaporation/condensation process. Thermal membranedistillation is an absolute filtration process where reverse osmosis isa relative filtration process.

Unfortunately, thermal distillation across a hydrophobic membrane hasnot been a practical approach for separating fresh water from saltwater. This is due to a major problem called water-logging. Salt watermigrates into the pores of the hydrophobic membrane and causes the freshwater and the salt water to make contact. In other words, the pores fillup with water thereby destroying the vapor barrier needed for theevaporation/condensation process.

The length of time it takes for water-logging to occur depends upon anumber of variables. Typically, it only takes a few days forwater-logging to occur and for the distillation process to stopcompletely. Although the membrane's vapor barrier can be re-establishedby draining the water from the cells and blowing the liquid out of themembrane pores with compressed air, having to do so every four to sixdays because of water-log greatly decreases the commercial applicabilityof thermal membrane distillation.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide improved thermalmembrane distillation.

Another object of the invention is to provide improved thermal membranedistillation utilizing a composite membrane composed of lyophobic andlyophilic layers.

Another object of the invention is to provide an improved thermalmembrane distillation process which provides continuous distillateproduction over prolonged periods of time.

Another object of the invention is to provide a multiple-layeredmembrane for thermal membrane distillation which is not subject toliquid intrusion into the pores of the lyophobic layer with continuoususe.

In accordance with the present invention, thermal membrane distillationis accomplished using a composite membrane separating the distillandfrom the distillate. The composite membrane comprises a thin microporouslyophobic layer and a thin lyophilic layer, with the lyophilic layeradjacent to the distilland and the lyophobic adjacent the distillate. Inapplications such as desalination of salt water, water-logging isprevented and continuous evaporation/condensation occurs within thepores of the hydrophobic layer.

The proper selection of pore sizes for both the lyophobic and lyophiliclayers is important in maximizing the distillate production rate throughthe composite membrane for a given temperature and temperaturedifference. Generally, higher distillate production rates have beenobserved in composite membranes with the smaller pore sizes in thehydrophilic layer than in the hydrophobic layer, in the case of saltwater distillation. Best results have occurred with the hydrophobiclayer having a mean pore size of less than 0.5 μm diameter. Thehydrophilic layer, desirably, should also have a mean pore size of lessthan 0.5 μm in diameter. It has also been found that the hydrophiliclayer can be non-porous. Optimum pore sizes for systems designed forpurposes other than water desalination may differ.

Examples of suitable porous hydrophobic materials for the compositemembrane of the present invention include polytetrafluoro-ethylene(PTFE) such as DuPont's "Teflon" and polyvinylidene fluoride (PVF₂).Suitable hydrophilic materials include cellulose acetate, mixed estersof cellulose, and polysulfone. Other suitable porous hydrophobic andhydrophilic materials can also be used.

PTFE is the most hydrophobic polymeric membrane material known. PVF₂ issomewhat less hydrophobic, but has also been shown to perform well.

Cellulose acetate membranes and membranes of mixed esters of cellulosehave water swelling properties, i.e., they expand when wetted with wateror other solvents. It is thought that the pores, especially those ofsmall diameter, may completely close on swelling and that the liquidtransport through the membrane does not require pores but migratesthrough the molecular structure. Polysulfone membranes swell very littleupon water wetting, therefore, their pore structure remains almostunchanged upon wetting.

Both the water-swelling hydrophilic membrane and the non water-swellinghydrophilic membrane in combination with a hydrophobic membrane preventthe water-logging phenomenon of prior art systems.

The composite porous membrane can be formed by clamping thehydrophobic/hydrophilic layers closely together to form a cell with asuitable support backing to maintain the integrity of the compositemembrane. In accordance with another aspect of the invention, thecomposite membrane is formed by coating one of the layers on the other.For example, a hydrophobic membrane can be used as a substrate uponwhich a hydrophilic layer is formed.

In accordance with another aspect of the invention, a plurality ofcomposite porous membranes are arranged in multiple stage cellconfigurations. A main advantage of multiple stage cell configurationsis the conservation of heat energy. In a multiple stage cell, multiplecomposite membrane system, the heat rejected from one stage is used toprovide the temperature gradient across another stage. The optimumnumber of stages is reached when the operating temperature of anyadditional stages is so low that the distillate production rate isuneconomically low.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged sectional view of a conventional, single-layer,hydrophobic porous membrane distillation device;

FIG. 2 is an enlarged sectional view of a compositehydrophilic/hydrophobic membrane in accordance with the presentinvention;

FIG. 3 is an exploded sectional view of a multiple-stage-cell, multiplecomposite membrane distillation system, in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a sectional enlargement of a single hydrophobic membrane 10separating a distilland 12 such as salt water, and a distillate 14 whichis fresh water. A pore 16 extends across the hydrophobic membrane 10.Initially, due to the hydrophobic nature of the membrane 10, neither thesalt water 12 nor fresh water 14 penetrates within the gaseous region 15of pore 16. Two convex-shaped liquid-vapor surfaces 18 and 20 aredefined at each end of pore 16.

Nonequal equilibrium states exist at the two interfaces 18 and 20 if thedistilland 12 has a higher vapor pressure than the distillate 14. Aspreviously explained, pure water evaporates into the capillary pore 16and the vapor is condensed at the vapor/liquid interface 20 on thedistillate side 14 in an effort to establish local vapor pressureequilibrium. The result is the creation of a net flux of water vaporacross the pore 16.

In distillation apparatus using the single hydrophobic membrane 10 ofFIG. 1, after several days the vapor barrier across the pores isdestroyed and hence the distillation process through the membrane,ceases. This is due to water-logging. The distilland liquid-vaporinterface 18 migrates in a direction toward the distillate side 14, asindicated by 18' and 18", until eventually it intersects theliquid-vapor interface 20 on the distillate side of the membrane 10.This destroys the gaseous barrier needed for the distillation process tooccur.

It is believed that the reasoon that water-logging occurs is due to theeffect of the dissolved salt in the distilland 12. The saline solutionchanges the hydrohobic property of the membrane 10. More particularly,it is believed that the saline solution changes the hydrophobic propertyof the membrane 10 where the liquid-vapor interface 18 contacts the sidewall 22 of the pore 16. As a result, the salt water, rather than beingprevented from filling the pore, is able to migrate within the pore,eventually completely filling it with liquid.

A possible explanation for the alteration of the normally hydrophobicproperty of membrane 10 follows.

During distillation, for a given pressure, evaporation takes place atapproximately a constant temperature. Addition of heat increases thekinetic energy of some liquid molecules enough to overcome theevaporation energy barrier. This heat is commonly known as the latentheat of evaporation. The latent heat of evaporation is a result ofmolecular attraction of the molecules which are tightly packed in theliquid phase. There is an energy barrier at the interface between thevapor and the liquid created by an unbalanced attracting surface forcefor vapor molecules onto the interface surface. The very short mean freepath (average molecular distance) on the liquid side compared with thevapor side creates the unbalanced forces at the interface. This is thephenomenon which creates surface tension.

It is possible to change the evaporation (or condensation) energybarrier by changing the surface curvature at the liquid-vapor interfacethereby requiring less kinetic energy by water molecules to escape fromthe liquid into the vapor. For a convex liquid surface, the greater theconvexity, the greater the evaporation rate for a given temperature.Further, a convex liquid surface will have a greater rate of evaporationthan a concave liquid surface. Conversely, the rate of condensation isless on a convex liquid surface than on a concave liquid surface.

The effect of the degree of curvature on evaporation rates can be seenin the formation of water droplets at a given temperature. Small drops,having greater curvatures, require higher vapor partial pressure toreach equilibrium with the surrounding gas mixture. As an example, thedistribution of different drop sizes existing in a cloud formation orcloud chamber will shift to larger drop sizes with time. The bulktemperature and vapor pressure is the same for all sizes of droplets.Due to the required higher equilibrium vapor partial pressure for smalldrops, they will evaporate to increase the bulk vapor pressure of thesurrounding gas mixture. This, however, causes a condensation to takeplace on the larger drops since their equilibrium vapor partial pressureis lower, due to their "flatter" outside curvature.

For a period of time the smaller sized drops disappear and the largesized drops become larger. In a rain cloud, as the size of a dropincreases, the gravitational force becomes larger than the air currentbuoyancy forces and rainfall begins.

In an analogous manner, it is believed that the dissolved salt in thedistilland 12 can alter the curvature of the liquid-vapor interface 18,thereby altering the evaporation energy barrier. Exactly what the natureof the modification is, is uncertain. But that it does appear to takeplace is suggested by another well known phenomenon, that of a dryingsalt water puddle.

In the drying process crystals or solids periodically precipitate at theperimeter to form concentric drying rings around the edge of the puddle.Usually the center of a drying puddle has a lower solids concentrationthan at the edge. A close examination of the edge of a puddle indicatesthat the curvature of the liquid surface at the edge of the puddlecontaining dissolved salts is considerably modified from a pure waterpuddle surface contacting edge. The evaporation energy barrier and thesalt concentration distribution at the edge of the liquid puddle arealso considerably changed from those of the homogeneous solution.

Since the surface tension is a function of the salt concentration, thesalt concentration in turn modifies the surface curvature. For example,increasing sodium chloride content increases the surface tension ofwater. The increased surface tension due to increased curvature at theedge creates two effects. First, it increases the rate of evaporation ofwater at the edge of the puddle. Secondly, it produces a higher saltconcentration at the edge while the center is left with the lowerconcentration of the homogeneous solution. The evaporation of the puddleincreases the burden of the surface tension as it attempts to hold tothe original boundary. Eventually, the original boundary cannot maintainits position and recesses to a new equilibrium position. When theboundary recesses, the salt precipitates at the boundary leaving behinda salt ring. The process repeats itself, forming multiple rings untilthe water is completely dried.

An analogous situation may be responsible for water-logging in thehydrophobic membrane pores. Salt concentration increases at the edge ofthe liquid-vapor interface 18 adjacent the pore wall 22 due to localsurface tension alterations caused by the effects of the hydrophobicmaterial on the salt water. This condition modifies the surface tensionthere, which in turn modifies the liquid-vapor interface curvature andincreases the rate of evaporation of water and causes salt precipitationto occur along the edge 22 of the liquid-vapor interface rendering thehydrophobic wall of the membrane hydrophilic. The original boundary istherefore not maintained, and the liquid-vapor interface 18 migrates toposition 18'. Eventually, migration causes the salt water interface tocollide with the fresh water interface, and water-logging occurs.

FIG. 2 is an enlarged sectional view of a composite porous membrane 24for thermal membrane distillation, in accordance with the presentinvention. The composite membrane 24 comprises a hydrophobic layer 26 towhich is closely joined a hydrophilic layer 28. The membranes must bepressed together closely enough to alter the surface tension effect atthe salt water/water vapor interface and, therefore, prevent saltprecipitation at the edge as will be explained subsequently. Thehydrophilic layer 28 is adjacent the distilland 12 and the hydrophobiclayer is adjacent the distillate 14. In the embodiment described, thedistilland is salt water and the distillate is fresh water.

As with the single hydrophobic distillation membrane 10 of FIG. 1,evaporation and condensation take place within micropores 29. Thehydrophilic layer 28 can also be micro-porous, but, because of theaffinity of water to the hydrophilic material of layer 28, the pores 30of the hydrophilic layer 28 fill by capillary action with salt water.With a vapor pressure gradient across the composite membrane 24 a netevaporation takes place at the salt water/vapor interface 32 and a netcondensation occurs at the fresh water/vapor interface 34. However,unlike the case of the single hydrophobic distillation membrane,water-logging does not occur within the composite membrane 24. It hasbeen found, as explained previously, that the hydrophilic layer need notbe porous. Nor is there a requirement that where the hydrophilic layeris porous, that the pores be aligned with the hydrophobic pores, asshown in FIG. 2.

The vapor transport rate across the pores 29 is dependent on the vaporpressure gradient across the pore cavity, i.e., on the vapor pressuredifference at the evaporating liquid/vapor interface 32 and thecondensing vapor/liquid interface 34. The vapor pressure is a functionof the temperature, salt concentration, and surface curvature.Therefore, all three properties must be considered in creating thedesired vapor pressure gradient across the composite membrane 24.

The reason that the composite hydrophilic/hydrophobic membrane does notexhibit water-logging is not known for certain. However, it is believedthat the hydrophilic layer adjacent the hydrophobic layer alters theliquid/vapor interface 32 in such a way that salt water 12 does notintrude within the pores 28. The hydrophilic layer 28 may interact withthe hydrophobic layer 26 to change the surface curvature of theliquid/vapor interface so that the salt concentration does not increaseat the edge of the pore. Both the hydrophilic and hydrophobic layers areneeded to accomplish continuous water distillation by a thermal membranedistillation process and to overcome the water log problem.

The proper selection of pore sizes for both the hydrophobic andhydrophilic layers of the composite membrane is important for maximizingdistillate production and insuring uninterrupted flow. The pore sizes ofthe hydrophobic layer should be chosen to withstand at least thehydraulic pressure exerted by the liquids on either side of themembrane. That is, the bubble pressure across the hydrophobic layer mustexceed, at a minimum, the absolute hydraulic pressure. The vaporpressure in the pore may be much lower than atmospheric pressure,therefore the bubble pressure must exceed the absolute hydraulicpressure rather than simply the gauge hydraulic pressure.

Bubble pressure is defined as the required pressure to overcome thecapillary surface tension. This is directly related to pore diameter.Since the porous hydrophobic material has a distribution of porediameters, the maximum absolute pressure on either side of thehydrophobic membrane must not exceed the bubble pressure for the largestpores. Because capillary forces are greater for smaller pores, bubblepressure increases as pore size decreases.

Best results have been obtained with mean pore size for the hydrophobiclayer of less than 0.5 μm for PTFE for desalination of salt water.Preferably, the mean pore size of the hydrophilic layer 28 is smallerthan the mean pore size of the hydrophobic layer 26. Generally, bestresults have been obtained with mean pore size less than 0.5 μm for thehydrophilic layer as well. The important objective of selecting thehydrophilic layer pore size is to alter the liquid-gas interface 32 atthe pores 29 of the hydrophobic layer to prevent migration of thedistilland into the hydrophobic pores to prevent water-logging. It hasalso been found that non-porous hydrophilic materials can also be usedin the composite membranes of the present invention.

The composite membrane should be made as thin as possible so that thevapor transport distance across the membrane is as short as possible. Apractical limitation on thinness is that it must be sufficiently strongthat it doesn't tear or rupture. Typical thicknesses for the hydrophobiclayer are about 1.5 mils for PTFE and about 2 to 6 mils for PVF₂.

Composite hydrophilic/hydrophobic membranes were tested using a PTFEhydrophobic membrane (Millipore "Fluoropore" filter--type FG) with amean pore diameter of 0.2 μm. Composite hydrophilic/hydrophobicmembranes were tested with hydrophilic mean pore sizes of 0.2 μm, 0.1μm, 0.5 μm, and 0.025 μm (Millipore "MF" filter, types GS, VC, VM, andVS, respectively, which are made of a mixture of cellulose acetate andcellulose nitrate).

The large mean pore diameters of 0.2 μm and 0.1 μm used for thehydrophilic layer used in the composite membrane system did notsignificantly differ in performance, but the composite membranes withthe 0.05 μm and 0.025 μm hydrophilic mean pore sizes did show thepositive influence of selective pore size diameter.

Experiments were performed to test and compare the performance of asingle 0.2 μm mean pore size PTFE hydrophobic membrane with a compositemembrane comprising a cellulose-acetate/cellulose-nitrate hydrophilicmembrane, also with a mean pore size of 0.2 μm, laminated with a PTFEmembrane with an identical mean pore size. The salt content used in thewater in these experiments was at least 30 grams in 1000 grams of water.Tests were conducted with the salt water heated to both 160° F. and 130°F. for comparison.

The results show that the single hydrophobic membrane has a rapiddistillate production rate drop off in the 160° F. case and stoppedcompletely in 4 days. At the lower temperature of 130° F., the flowthrough the single membrane stopped in 2 days. In contrast, thecomposite membranes ran for 20 days and still showed no signs ofstopping, at which time the experiment was discontinued.

The composite membrane of the present invention functions effectivelyeven at very low temperatures as long as the vapor pressure gradientexists from the distilland to the distillate. The concentration ofdissolved salts in the distilland increases with time asevaporation/condensation takes place which eventually causes theprecipitation of salts, unless some high salt concentration distillandis drained off and replaced with less concentrated distilland. If thecomposite membrane causes the salt to concentrate in the center of theliquid/vapor interface 32, or at least not cause salt to precipitate atthe edge of the liquid-solid interface, then the salt concentration,although high at the interface, can still be diffused back into the bulkdistilland solution rather than precipitate at the interface and causewater-logging.

Most of the trapped gas in the hydrophobic pores may be absorbed by thecondensate water, especially at low temperature operation. This createsa partial vacuum such that the equilibrium at the liquid-vapor interface32 is changed. The partial vacuum increases the vapor diffusion rate butalso reduces the condensation rate at the liquid/vapor interface 34 atthe distillate side.

One of the essential element in water desalination is the bestutilization of available energy. The cell in a composite thermaldistillation membrane system can be operated at atmospheric or higherwater pressures at temperatures as low as 120° F. Consequently, manysources of waste heat from industrial plants and also from home uses,for example, may be used as energy for creating the necessarytemperature differential for thermal distillation to occur across thecomposite membrane.

In any distillation process, enough energy must be supplied to overcomethe latent heat of evaporation, which for water is about 900 Btu/lb.However, some of the heat used for evaporation of the water on thedistilland side of the composite membrane can be recovered uponcondensation of the distillate. In a thermal membrane distillationsystem, the temperature drop from the saline water side to the freshwater side desirably exceeds about 2° F.

FIG. 3 is an exploded, sectional view of a multiple stage celldesalination system 50 with three cell stages 52, 54 and 56.Additionally, there is a pre-heat cell stage 58. Each cell stage, 52,54, and 56 includes two ring members 59 and 60, which sandwich andsupport a composite membrane 61 comprising a hydrophilic membrane 62, ahydrophobic membrane 64, and a support screen 66. Individual cell stagesare separated by stainless steel plates 67 which pass waste heat to theadjacent, lower temperature cell stages.

Support screen 66 is made of a material, such as stainless steel, whichis chemically inert to salt water. It is necessary to support the thincomposite membrane 61 to be able to hold the hydrophobic membraneagainst the hydrophilic membrane and also to prevent rupturing. Thescreen is attached along its periphery by a stainless steel loop 68which fits into a recess 70 of ring member 60.

An end cap 72 completes one end of the multiple stage cell system 50.Another end cap 74 seals the other end by engagement with another ringmember 76. End cap 74, ring member 76, and stainless steel plate 67define the pre-heat cell stage 58. O-rings 78 are provided to maintainthe integrity of the pre-heat cell 58, as well as the membrane cellstages 52, 54 and 56. The cell stages 52, 54 and 56 as well as thepre-heat cell 58, and end plates 72 and 74 are clamped together bysuitable means, not shown for clarity.

For sake of clarity, the fresh and salt water sides of each of the cellstages 52, 54 and 56 are designated with the letters "F" and "S",respectively. Thus, cell stage 52 has a fresh water side F1 and a saltwater side S1, cell stage 54 has sides F2 and S2, and cell stage 56 hassides F3 and S3. Conduits 82, 84 and 86 are provided from the freshwater side of cell stages 52, 54 and 56, respectively, for recoveringthe condensed distilled water.

A source of salt water to be desalinated is shown at 88. It should beunderstood that while salt water is the distilland in this particularembodiment, any aqueous solution containing dissolved minerals orparticles can be distilled using the multiple-stage-cell apparatus 50.This includes, for example, muddy water, sewage, and aqueous solutionsof industrial wastes.

A heat source is provided at 90. Desirably, waste heat is used, but aheater can be used if waste heat is not available. In themultiple-stage-cell system 50, each successive cell stage is cooler thanthe preceding cell stage, with cell stage 52 being the hottest. Theremaining distillation cell stages 54 and 56, and pre-heat cell stage 58are each successively cooler. Of course, for each composite membrane 61within each cell stage, there is a temperature difference maintainedacross it.

The flow of salt water being distilled will now be explained. Salt waterenters the pre-heat cell 58 through conduit 92. The salt water iselevated in temperature by heat transmitted through stainless steelplate 67 from the fresh water side F3 of cell stage 56.

Some of the salt water passing through pre-heat cell stage 58 is passedby pump 94 through an outlet conduit 96, through a heat exchanger 98where waste heat is dispelled, to maintain the end of the system 50 ascool as possible. The remainder of the salt water passes out of pre-heatcell stage 58 through conduit 100 to the salt water side S3 ofdistillation cell stage 56. Additional salt water is added to pre-heatcell stage 58 as required.

Salt water entering the S3 side of cell stage 56 is heated by the freshwater side F2 of cell stage 54 through stainless steel plate 67.Desalinated fresh water passes out from the fresh water side F3 of cellstage 56 through conduit 86.

Partially distilled salt water from the S3 side passes though conduit102 to side S2 of cell stage 54, where it is further heated by the freshwater side F1 of cell stage 52 through stainless steel plate 67.Distilled fresh water passes out of F2 through conduit 84 after passingthrough the composite membrane.

The partially distilled salt water, now having a higher salinitycontent, then passes out of side S2 of cell stage 54 through conduit 104to heat source 90. There it is further heated and is pumped to the saltwater side S1 of cell stage 52 through conduit 106 by pump 108. Saltwater flowing to cell stage 52 passes through a filter 110 and a flowmeter 112.

Distilled water from side F1 of cell stage 52 flows out of the cellthrough conduit 82. Remaining salt water from side S1 passes out ofconduit 114 where it is re-heated and mixed with salt water from S2.Some highly concentrated salt water is drained at conduit 115 tomaintain the salt concentration at a desired level. Of course, saltwater can be drained at any cell stage to adjust the salt concentrationas desired.

It can be seen that each cell stage is "driven" by heat from thepreceeding cell stage. Additionally, since the fresh water sides F1-F3give off heat to the salt water side, the vapor pressure of the freshwater side is maintained below the vapor pressure of the salt water sideof that cell stage by maintaining a sufficient temperature difference.Thus, a vapor pressure gradient is established across each of thecomposite membranes 61 which is required for distillation to take place.

In an actual embodiment each of the ring members 59 and 60 and the endplates 72 and 74 are made of a polycarbonate plastic. Of course, ofprimary importance in the selection of this material, is the ability towithstand the corrosive effects of warm salt water solutions.

The hydrophilic membranes 62 in distillation system 50 are madetypically of cellulose acetate/cellulose nitrate mixture such as typeNo. MF-VM made by Millipore. The mean pore diameter for this membrane is0.05 μm. The hydrophobic membranes 64 are PTFE, such as Millipore's"Fluoropore" filter, Type FG, which has a mean pore diameter of 0.2 μm.

Once the system reached a steady state condition, the output of purewater from distillation system 50 was 10 pounds of water per square footof composite membrane 61 per day for cell stage 56; 12 pounds for cellstage 54; and 7 pounds for cell stage 52. Salt water was drained at 6.6pounds of water per square foot per day of membrane area.

Representative temperatures within each cell stage are:

Preheat: 75° F.

F3: 95° F.

S3: 115° F.

F2: 126° F.

S2: 144° F.

F1: 158° F.

S1: 171° F.

To clamp the hydrophobic membrane 64 against the hydrophilic membrane 62and also to protect against contamination in the event of rupture of acomposite membrane 61, the fresh water sides F1-F3 are maintained at ahigher pressure than the respective salt water sides S1-S3 of each ofthe cell stages.

It is preferable in case of the multiple stage cell configuration ofFIG. 3 to have higher concentration saline solutions in the highertemperature cells. It was found that higher temperatures are required tomaintain acceptable distillate production rates from the moreconcentrated distilland solutions. This is determined by thethermodynamic states of the solution. In general, it is preferred toarrange brine feed so that the salt concentration increases with celltemperature. Thus, the salt concentration is greatest at S1 and lowestin S3.

In multiple stage cell systems, the energy used for fresh waterproduction is reduced approximately as the inverse of the number ofcells. For example, a three-stage cell system uses about one-third ofthe energy required to produce fresh water in a single-stage cellsystem. Typically, in a multiple stage cell system with salt watertemperature of 180° F. in the hottest cell, the required energy toproduce pure water will be as low as 180 Btu/lb. which is one-fifth theenergy necessary to evaporate water.

Commercial stills also use multiple stage evaporation techniques, butlimiting consideration for the optimum number of conventionalevaporation stages is the pressure difference between evaporator stages,the pressure difference becoming smaller with the addition of stages. Onthe other hand, with the multiple-stage composite membrane cell system,such as shown in FIG. 3, the limiting consideration is the temperaturedifference across the cell stages.

When brine attains a sufficient concentration, it has a commercial valueas feedstocks to other process industries. High concentration salinesolutions may be taken out of the system and reused. Salt brineconcentrations as high as 25% have been achieved utilizing the compositemembrane cells of the present invention.

What is claimed is:
 1. A method of thermal membrane distillation fordistilling aqueous solutions comprising:providing a thin poroushydrophobic layer separating an aqueous solution from fresh water;providing a sufficient temperature gradient across a porous hydrophobiclayer so that liquid water evaporates on the aqueous solution side ofthe pores of the hydrophobic layer and condenses as pure water on thepure water side of the pores of the hydrophobic layer; and providing ahydrophilic layer on the aqueous solution side of the hydrophobic layerfor preventing aqueous liquid intrusion within the pores of thehydrophobic layer.
 2. The method of thermal membrane distillation as inclaim 1 wherein said hydrophilic layer is porous.
 3. The method ofthermal membrane distillation as in claim 2 wherein the mean porediameter of the hydrophilic layer is less than the mean pore diameter ofthe hydrophobic layer.
 4. The method of thermal membrane distillation asin claim 3 wherein the hydrophobic layer is polyvinylidene fluoride. 5.The method of thermal membrane distillation as in claim 2 or 4 whereinthe hydrophilic layer is cellulose acetate.
 6. The method of thermalmembrane distillation as in claim 2 wherein the average pore sizediameter of the hydrophilic layer is less than 0.5 μm.
 7. The method ofthermal membrane distillation as in claim 1 wherein the average poresize diameter of the hydrophobic layer is less than 0.5 μm.